This invention pertains generally to radar fuzing systems and more particularly to an improved mixer/correlator for use in such systems.
As is known in the art, radar fuzing systems, especially those designed for use on board missiles where packaging volume is at a premium, operate at relatively low power levels. Therefore, such systems usually are operated in a continuous wave mode in order to maximize energy on target and are arranged to utilize some kind of binary coding technique (such as that known as polyphase coding) to obtain the requisite target range information. The most common type of polyphase coding is that wherein the phase of the radio frequency (RF) carrier is alternated between 0 and 180 degrees in accordance with an applied binary code. The requisite phase switching is usually accomplished by so-called P-I-N diode phase shifters. Upon reception range information is obtained by correlation processing which is usually performed after the received signals have been downconverted to intermediate frequency (IF) signals or to baseband video signals. Such an approach generally implies a receiver with a relatively high noise figure.
Correlation of any polyphase modulated received signal is achieved by comparing such signal with a delayed replica of the transmitted polyphase code. The time delay corresponds to the desired range to be instrumented at any particular time. The performance of any such system (meaning the accuracy with which range is measured and the degree to which out-of-range clutter returns are rejected) is determined in the first instance by the by the accuracy with which the transmitted RF carrier signal is modulated. Such modulation in turn is determined by the switching speed of the diode phase shifters. Even though high speed P-I-N diodes may be used because of the relatively low power level of the RF carrier signal, the drive circuits for such diodes are usually complex, are power consumptive, and require a substantial packaging volume.